(1) Field of the Invention
The present invention relates to a semiconductor tester, semiconductor integrated circuit and semiconductor testing method for checking the output voltages from a semiconductor integrated circuit.
(2) Description of the Prior Art
With the development of liquid crystal display panels (to be referred to hereinbelow as LCD panels) into high definition, liquid crystal display driver ICs (to be referred to hereinbelow as LCD driver ICs) tend to have a greater number of outputs with a greater number of tonal voltage levels. In general, the tonal voltage levels for a LCD driver IC are determined based on the resistive division ratios or capacitive division ratios to the voltage applied from the reference power input terminal of a gamma control resistive circuit and gamma control capacitive circuit incorporated in the device as a reference voltage generating circuit, and the more the tonal levels there are, the more division ratios are broken down into smaller sections.
In order to achieve display of multi-leveled tones, LCD drivers incorporate digital/analog converters (to be referred to hereinbelow as D/A converters) for converting the input digital image data corresponding to the number of tonal voltage levels into analog tonal voltage outputs and output the tonal voltages. For example, a LCD driver IC for 256 tones of display incorporates 8-bit D/A converters.
In the testing of such a multi-output, multi-tonal LCD driver IC, it is normally checked whether each D/A converter converts all the tones of digital input image data into correct levels of voltage and whether the tonal voltages corresponding to an individual tone are converted equally between all the D/A converters.
The checkout of the IC drivers in this case is generally based on three criteria, namely, the maximum positive and negative deviations of the tonal output voltages from all the output terminals with respect to the ideal voltage for each tonal output voltage level, and the variation of the tonal output voltages from all the output terminals (the sum of the maximum positive and negative deviations among all the output voltage terminals). For example, a device of which the maximum positive deviation and maximum negative deviation of actual tonal output voltages from the ideal tonal output voltage fall outside of xc2x130 mV and the variation among all the output terminals falls outside of about 35 mV should be rejected as a defective. Thus, the checkout demands a markedly high precision of measurement.
FIG. 1 is a chart showing one example of the tonal voltage output test result. In the example shown in this chart, a product is determined to be non-defective if the absolute values of the maximum positive and negative deviations are equal to or lower than 30 mV and the variation among all the output terminals is equal to or lower than 35 mV.
In the testing of LCD driver ICs, input terminal leakage test, functional operation test, electric current consumption test, etc., are also implemented other than the above-described tonal voltage output test. However, the tonal voltage test takes up to about 80% of the total testing time of all these test items. Further, the trend toward increased number of outputs and tones of LCD driver ICs tends to further increases the time required for tonal voltage output test.
This means that reduction of the testing time taken for the tonal voltage output test is the most critical factor in reducing the testing time and cost of LCD driver ICs.
In order to measure the tonal output voltages accurately, it is preferred that the semiconductor tester should have as many voltmeters of a high precision as the number of the tonal output terminals. But if the device is so configured, it becomes too expensive and large-sized. Therefore, from economic and other view points, the number of voltmeters incorporated in a semiconductor tester has been limited to one to a few.
In order to implement the tonal voltage output test with high accuracy in a short time, various methods have been proposed in consideration of economy.
For example, according to the invention of Japanese Patent Application Laid-Open Hei 10 No. 2935, a semiconductor tester is disclosed which comprises: a multiple number of sample hold circuits for sampling the multiple voltages to be tested from a test specimen such as an LCD driver IC or the like, at the same time, in a single operation; a multiplexer for sequentially selecting the multiple sample hold circuits, one by one, so as to output the selected output; and a checkout circuit for checking whether the voltage to be tested, sequentially output from the multiplexer circuit, is suitable based on the associated upper and lower limit voltages.
This disclosure claims that it is possible to reduce the time taken for tonal voltage output test while keeping the precision of measurement since all the tonal output terminals can be checked by using a limited number of voltmeters of high precision and rapidly switching them.
However, this disclosure does not mention the testing of the variation among output terminals. In this case if the test of the variation among output terminals is implemented, this would be performed by storing the voltages from all the output terminals, extracting the maximum and minimum values among the output terminals, calculating the difference between the extracted maximum and minimum values, and comparing the result with the associated reference value.
However, according to the invention described in Japanese Patent Application Laid-Open Hei 10 No. 2935, since the voltages output from the device to be tested are measured terminal by terminal, the upper and lower limit voltages as the reference values need to be reset for every tone or every test voltage level.
The number of voltage measurements is the product of the number of voltage output terminals of the device to be tested and the number of output voltage tones. That is, a device with many tones and many outputs needs an enormous time for testing, resulting in unfeasibility. For an LCD driver IC having 480 outputs and 256 tones, for example, because two output voltages, positive and negative, are needed for each tone, the number of measurements is calculated as the product of the number of outputs, 480, the number of tones, 256, and 2 for positive and negative, which amounts to about 250,000 checks. If one measurement needs about 0.1 millisecond, the test will take up to 25 seconds for each device.
Other than the time for switching the high-precision voltmeter to each tonal voltage terminal, the time needed for one measurement is also largely dependent on the voltage measurement time for actually measuring each tonal voltage and the calculation time for calculating the amount of output variation from terminals. Therefore, even if the switching time of the high-precision voltmeter to each tonal voltage terminal can be made short, it is impossible to sharply reduce the time of measurement. So there remains the problem of reducing the measurement time as long as the test is performed by measuring all the output voltages.
It is therefore an object of the present invention to provide a simple and low-cost semiconductor tester, semiconductor integrated circuit and semiconductor testing method, which make it possible to perform rapid check of output voltages from a multiple number of output terminals of a semiconductor integrated circuit.
In order to achieve the above object, the present invention is configured as follows:
In accordance with the first aspect of the present invention, a semiconductor tester for checking the output voltages of a semiconductor integrated circuit having a multiple number of output terminals which output predetermined voltages, includes: a judgement means which, with reference to the mean voltage value of the voltages output from all the output terminals of the semiconductor integrated circuit, judges the suitability of the output voltage from each of the output terminals.
In accordance with the second aspect of the present invention, the semiconductor tester having the above first feature is characterized in that the judgement means comprises: a multiple number of resistors connected at their first ends to the respective output terminals and at their second ends to a common signal line; and a first comparing means which compares the potential difference between both ends of each resistor with a predetermined first reference voltage value.
In accordance with the third aspect of the present invention, the semiconductor tester having the above first feature is characterized in that the judgment means comprises a second comparing means for comparing the mean voltage value with a predetermined second reference voltage value.
In accordance with the fourth aspect of the present invention, the semiconductor tester having the above second feature is characterized in that the judgment means comprises a second comparing means for comparing the mean voltage value with a predetermined second reference voltage value.
In accordance with the fifth aspect of the present invention, a semiconductor integrated circuit having a multiple number of output terminals which output predetermined voltages, includes: a judgement signal output means which, with reference to the mean voltage value of the voltages output from all the output terminals, outputs signals representing the suitability of the output voltages from the output terminals.
In accordance with the sixth aspect of the present invention, the semiconductor integrated circuit having the above fifth feature is characterized in that the judgement signal output means comprises: a multiple number of resistors connected at their first ends to the respective output terminals and at their second ends to a common signal line; and a first comparing means which compares the potential difference between both ends of each resistor with a predetermined first reference voltage value.
In accordance with the seventh aspect of the present invention, the semiconductor integrated circuit having the above fifth feature is characterized in that the judgment signal output means comprises a second comparing means for comparing the mean voltage value with a predetermined second reference voltage value.
In accordance with the eighth aspect of the present invention, the semiconductor integrated circuit having the above sixth feature is characterized in that the judgment signal output means comprises a second comparing means for comparing the mean voltage value with a predetermined second reference voltage value.
In accordance with the ninth aspect of the present invention, a semiconductor testing method of checking the output voltages of a semiconductor integrated circuit having a multiple number of output terminals which output predetermined voltages, includes the step of judging the suitability of the output voltage from each of the output terminals, with reference to the mean voltage value of the voltages output from all the output terminals of the semiconductor integrated circuit.
In accordance with the tenth aspect of the present invention, the semiconductor testing method having the above ninth feature is characterized in that the step of judgement includes a first comparing step for comparing the potential difference between both ends of each of a multiple number of resistors with a predetermined first reference voltage value, the multiple number of resistors being connected at their first ends to the respective output terminals and at their second ends to a common signal line.
In accordance with the eleventh aspect of the present invention, the semiconductor testing method having the above ninth feature is characterized in that the step of judgement includes a second comparing step for comparing the mean voltage value with a predetermined second reference voltage value.
In accordance with the twelfth aspect of the present invention, the semiconductor testing method having the above tenth feature is characterized in that the step of judgement includes a second comparing step for comparing the mean voltage value with a predetermined second reference voltage value.
Next, the operation and effect of the present invention will be described.
In accordance with the semiconductor tester of the present invention, the output voltages of a semiconductor integrated circuit having a multiple number of output terminals which output predetermined voltages are checked with reference to the mean voltage value of the voltages output from all the output terminals. That is, this mean voltage value is used as the reference voltage value, based on which the suitability of the output voltage from each of the output terminals is judged. Therefore, it is possible to simply set the reference voltage value, which should be changed every time the voltage to be tested is changed for a different level of tone.
It is also possible to reduce the number of voltage sources needed when comparison with the reference voltage values is to be made.
Since the judgement means includes a multiple number of resistors connected at their first ends to the respective output terminals and at their second ends to a common signal line, the common line connected to the second ends of the resistors presents the mean value of all the output voltages, conforming to the Kirchhoff""s law. Therefore, the potential difference between both ends of each resistor can be detected easily as the deviation of the potential at the output terminal to which the first end of the resistor is connected, from the mean output voltage value.
Further, since a first comparing means which compares this deviation with the first reference voltage value as the permissible value of variation and determines if each deviation falls within the permissible range is provided, it is possible to detect any anomaly instantly if any of the output terminals has a defect.
Moreover, this first reference voltage can and should be constant throughout the voltage test when the permissible voltage width is constant for all the tones. Therefore, it is no longer necessary to switch the reference voltage, which would have had to be changed every time the voltage to be tested should have been changed depending on the level of tone.
In the semiconductor tester of the present invention, since the judgement means includes a second comparing means for comparing the mean voltage value with a predetermined second reference voltage value, it is possible to smoothly perform rough detection of the error or deviation from the voltage to be output from each output terminal without performing direct measurement of the output voltages from all the terminals.
The semiconductor tester of the present invention includes a judgement signal output means which, with reference to the mean voltage value of the voltages output from all the output terminals, outputs signals representing the suitability of the output voltages from the output terminals, this mean voltage value is used as the reference voltage value, based on which the suitability of the output voltage from each of the output terminals is judged. Therefore, it is possible to simply set the reference voltage value, which should be changed every time the voltage to be tested is changed for a different level of tone.
It is also possible to reduce the number of voltage sources needed when comparison with the reference voltage values is to be made.
In accordance with the semiconductor integrated circuit of the present invention, since the judgement signal output means includes a multiple number of resistors connected at their first ends to the respective output terminals and at their second ends to a common signal line and a first comparing means which compares the potential difference between both ends of each of the resistors with the first reference voltage value, the second ends of the resistors present the mean value of all the output voltages, conforming to the Kirchhoff""s law. Therefore, the potential difference between both ends of each resistor can be detected easily as the deviation of the potential at the output terminal to which the first end of the resistor is connected, from the mean output voltage value and is output to the outside of the semiconductor integrated circuit.
Further, since the first comparing means which compares this deviation with the first reference voltage value as the permissible value of variation and determines if each deviation falls within the permissible range is provided, it is possible to detect any anomaly instantly if any of the output terminals has a defect. Therefore, it is possible for an external device to easily detect anomalies of the output terminals.
Moreover, this first reference voltage can and should be constant throughout the voltage test when the permissible voltage width is constant for all the tones. Therefore, it is no longer necessary to switch the reference voltage, which would have had to be changed every time the voltage to be tested should have been changed depending on the level of tone.
Since the semiconductor testing method of the present invention includes the step of judging the suitability of the output voltage from each of the output terminals, with reference to the mean voltage value of the voltages output from all the output terminals of the semiconductor integrated circuit, this mean voltage value is used as the reference voltage value, based on which the suitability of the output voltage from each of the output terminals is judged. Therefore, it is possible to simply set the reference voltage value, which should be changed every time the voltage to be tested is changed for a different level of tone.
It is also possible to reduce the number of voltage sources needed when comparison with the reference voltage values is to be made.
Since the step of judgement includes a first comparing step for comparing the potential difference between both ends of each of a multiple number of resistors, which are connected at their first ends to the respective output terminals and at their second ends to a common signal line, with a predetermined first reference voltage value, the second ends of the resistors present the mean value of all the output voltages, conforming to the Kirchhoff""s law. Therefore, the potential difference between both ends of each resistor can be detected easily as the deviation of the potential at the output terminal to which the first end of the resistor is connected, from the mean output voltage value.
Further, since a first comparing means which compares this deviation with the first reference voltage value as the permissible value of variation and determines if each deviation falls within the permissible range is provided, it is possible to detect any anomaly instantly if any of the output terminals has a defect.
Moreover, this first reference voltage can and should be constant throughout the voltage test when the permissible voltage width is constant for all the tones. Therefore, it is no longer necessary to switch the reference voltage, which would have had to be changed every time the voltage to be tested should have been changed depending on the level of tone.
In the semiconductor testing method of the present invention, since the step of judgement includes a second comparing step for comparing the mean voltage value with a predetermined second reference voltage value, it is possible to smoothly perform rough detection of the error or deviation from the voltage to be output from each output terminal without performing direct measurement of the output voltages from all the terminals.